The present invention relates to a helmet which is designed to absorb more energy during an impact, particularly rotational energy, with the aim of reducing brain trauma.
It is known that helmets provide protection against brain injury when they absorb energy in an impact. However, the protection provided is sometimes not sufficient to protect against brain damage, especially if the impact is particularly severe. Attention has recently been focused on protecting the head during oblique impacts, in which both linear and rotational forces are experienced.
During a fall, the head has a combination of linear and rotational energy. Upon impact, this energy has to be transferred, and the time duration of the transfer is often under 10 milliseconds. The brain floats inside the skull in cerebrospinal fluid and can move independently of the skull to a certain degree. The brain will continue to move after the skull has come to rest or reversed direction. As the brain decelerates, strain and shear forces are created which can cause structural damage to the brain, and/or set off a pathophysiological cascade of chemical processes that can lead to neuron and glial death.
The object of a helmet is to reduce peak acceleration, which in turn reduces the strain and shear forces on the brain during an impact, limiting brain damage.
A problem of existing helmet designs is that the response time of the helmet system may be too slow to limit the rotational acceleration, and there may be no rotational energy absorption to prevent serious injury. The greatest increase in acceleration is generally in the first microseconds of an impact, and so fast response time is critical if the peak acceleration is to be reduced.
As described in WO 2011/139224, it is known to provide a helmet having an inner liner layer and an outer liner layer, the inner liner layer being rotatable within the outer liner layer. In this way, rotational acceleration of the brain can be reduced, because the outer liner layer of the helmet will rotate, ‘slipping’ with respect to the inner liner layer. The layers are held together by fixation members between the layers, which deform plastically or elastically to allow rotation when needed. However, the fixation members slow the response time of the helmet, and limit the maximum rotation between the layers. This reduces the effectiveness of the helmet.
The deformation of the fixation members is also the primary mechanism by which rotational energy is absorbed by the helmet. The helmet's capacity to absorb energy is therefore limited. If a fixation member breaks in a serious fall, then it will not be able to absorb any more energy after it has broken.
A further problem with existing helmet designs is that, to provide inner and outer ‘slip’ layers, either the overall size of the helmet must be increased, which may be uncomfortable and/or unsightly, or the amount of padding for absorbing the energy in radial impacts has to be reduced. It is clear from FIG. 1 in WO2011/139224 that a gap must be provided between the inner and outer liner layers, and increased performance in arresting rotational forces is therefore at the cost of decreased performance in arresting radial forces. Regardless of safety considerations, consumers will tend to prefer good-looking and comfortable helmets.
It is an object of this invention to provide an improved helmet which more effectively protects against both linear and rotational impacts, without the need for substantial extra bulk in the helmet.